This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.
Cellulose is the most abundant bio-based material on earth (more than 50% of the biomass). Cellulose occurs naturally in, for example, trees, cotton, bacteria, and algae. The cellulose fibers in wood are composed of amorphous and crystalline regions. These crystalline regions are rod-shaped, nano-sized particles remaining after the acid hydrolysis of amorphous regions of cellulose fibers. During the past decade, there has been a great interest in these Cellulose Nanocrystals (CNCs) due to their unique characteristics. CNCs have a high aspect ratio. CNCs are 100% renewable, very strong, stiff, resilient, and light weight. Therefore, the intriguing ability of CNCs to reinforce host materials and form a dense network leads to highly versatile, sustainable, and environmentally friendly polymer composites capable of enhancing mechanical and physical properties.
However, there are limitations and drawbacks, which are linked to their intrinsic physical properties including high moisture absorption of CNCs based materials, agglomeration of nanoparticles, difficulty in redispersing agglomerated particles, and incompatibility with hydrophobic polymers in nanocomposite applications. The incompatibility with hydrophobic polymers in nanocomposite applications limits the use of CNCs as reinforcing agents for thermoplastic composite applications due to the low compatibility of CNCs in organic media (both solvents and polymer matrices). Moreover, even if CNCs are relatively compatible, it is not easy to redisperse the agglomerated CNCs caused by the strong hydrogen bonding of the crystals into a solution or polymer matrix. The most optimal solution to this issue is surface hydrophobization of CNCs. The surface hydrophobization is achieved by the use of direct chemical modification of the enormous number of hydroxyl groups in the structure of CNCs. When carbodiimide, isocyanates, epoxides, acid anhydrides, acid halides, and alkyl halides are used as a covalent attachment of molecules, they react with the surface hydroxyl groups of cellulose to yield hydrophobic surfaces bearing hydrocarbon moieties that promote an excellent dispersion of CNCs in organic media. Moisture sensitivity and corresponding dimensional changes can be considerably avoided if the CNCs are fairly modified by hydrophobic attachments and there are strong interactions between the modified CNCs and polymer matrix. Functionalization of CNCs has already been much reported in the literature using (1) physical adsorption of some types of surfactants, (2) covalent attachment of molecules, and (3) grafting polymers at the surface of the CNCs.
Nonetheless, most of these techniques require the use of hazardous solvents and reactants which are toxic to human health and the environment, are complex and multistep synthetic pathways, and yield low modification. It is difficult for CNCs to react with organic reagents directly due to their poor solubility in the organic phase. CNCs in a water suspension need a solvent exchange with dimethyl sulfoxide (DMSO), dimethyl formamide (DMF), or other compatible solvent. The reactions also need special reagents such as acid chloride, acid anhydride, isocyanate or carbodiimide which can be difficult to handle and are expensive. Furthermore, process lines exposed to organic solvents that release high levels of volatile organic compounds (VOCs) as chemical gasses can pollute the air and have a harmful effect on human health. Nowadays, people are increasingly demanding products that emit lower levels of VOCs. Therefore, there has been a great effort from both academia and industry in the development of green technologies capable of changing the use of toxic organic solvents to safer alternatives such as solvent-free reactions or aqueous media reactions. Many research projects related to this issue aim to reduce solvent-derived environmental harm. Therefore, there is an unmet need for the development of safer, more economically feasible, and more ecofriendly methods, accompanied with higher grafting yields.
In addition, the direct grafting of large molecules such as long chain fatty acids at the surface of the CNCs may not provide sufficient grafting of the hydroxyl groups. There is a need for improved strategy to make new hydrophobilized CNCs having compatibility with hydrophobic polymers in nanocomposite applications.